The field of the invention is that of data transmission, in particular of the frequency division multiple access (FDMA) type, consisting in transmitting a frequency multiplex of channels.
In general, each channel of the multiplex is associated with a carrier frequency and with a data rate. In the present case, the carrier frequency and the data rate of each channel can be completely arbitrary.
More precisely, the invention relates to a digital method and to digital apparatus for filtering, decimating, and transposing into baseband a particular channel of a frequency multiplex. In other words, the present invention relates to a method and to apparatus serving to extract a particular channel referred to as the xe2x80x9cchannel of interestxe2x80x9d from a frequency multiplex of channels, and to transpose it into baseband, and to do so while reducing the oversampling factor of said channel of interest.
In conventional manner, the operations of filtering, decimation, and transposition are implemented in a demodulator capable of receiving a frequency multiplex, and they are performed prior to other operations, for example operations such as filtering, reception, interpolation, synchronization, decoding, etc.
Also in conventional manner, it is assumed below in the present description that the frequency multiplex has already been transposed to an intermediate frequency or indeed is already in baseband. It is recalled that when a multiplex is in baseband that does not necessarily mean in any way that the channel of interest within said multiplex is itself in baseband. Consequently, it is clear that the invention (which seeks to bring the channel of interest into baseband) is applicable to both of the above cases.
The present invention relates solely to digital type processing (filtering, decimating, and transposing). In order to take maximum advantage associated with digital technology (reproducibility, precision, flexibility of design rules, . . . ), it is desirable for certain tasks that have traditionally been performed in the analog domain to be performed in completely digital manner.
This means that the frequency multiplex as a whole has already been digitized (sampled and subjected to analog-to-digital conversion). In other words, the entire frequency multiplex has been sampled at a rate that is relatively high because its band is still complete (for example the sampling rate may lie in the range 40 MHz to 60 MHz).
Since the channel of interest (the channel that is to be extracted and transposed into baseband) is of narrow width compared with the sampling frequency, it is desirable to reduce the oversampling on said channel of interest to a moderate value (4 to 8 samples per symbol, for example). Typically, the data rates processed lie in the range 10 kbit/s to 500 kbit/s, for example.
This reduction of oversampling is performed by an operation known as xe2x80x9cdecimationxe2x80x9d which consists in selecting one out of every n samples. The factor n is referred to as the xe2x80x9cdecimationxe2x80x9d factor.
In conventional manner, the operation of decimation is preceded by a filtering operation. Decimation produces aliasing. Efficient filtering purges the frequency band in those locations where information-carrying spectrum components are going to be aliased.
It is also desired to transpose the channel of interest into baseband so as to make subsequent processing possible (filtering, reception, interpolation, synchronization, decoding, etc.). The channel of interest is located at an arbitrary frequency within the multiplex and indeed there is no guarantee that the multiplex is itself centered on zero frequency.
A digital demodulator can solve the problem of frequency divided channels by using a decimating filter that has a tree structure. Each stage of the structure has two branches, each branch performing filtering on a sub-band equal to half the total band it receives, followed by decimation by two. The tree structure makes it possible to extract simultaneously as many distinct channels as it possesses outputs, i.e. 2E distinct channels where E is the number of stages. It is also important to observe that the tree structure does not require any specific transposition means, since the filtering and decimation performed at each stage bring the carriers of the various channels progressively towards zero frequency.
Unfortunately, such a tree structure suffers from the major drawback of being unsuitable for use with an arbitrary multiplex. A tree structure requires the channel carriers to be distributed in a very specific manner within the multiplex. More precisely, each channel carrier must occupy an initial position within the multiplex such that at each stage it is to be found exactly within one of the two filtering sub-bands. It will be understood that if at the input to a given stage (i.e. at the output from the decimator of the preceding stage) a carrier lies at the boundary between the sub-bands associated with the two filters of the stage (overlap of transition zones), then it runs the risk of being conserved by neither of the two filters.
Furthermore, the tree structure is designed to extract all of the channels of the multiplex and such a structure is therefore excessive when only one channel is to be extracted from a plurality of channels.
In the state of the art, solutions are also known relating solely to the pair of operations comprising filtering and decimation. Those prior solutions recommend avoiding only one step of filtering for high rates of decimation. Such an approach would require a large fraction of the frequency band to be filtered so as to leave room for the large amount of aliasing produced by high rates of decimation. Unfortunately, such a constraint means that the transition zone of the filter performing the rejection operation is narrow, which can only be achieved by a digital filter that is defined by a large number of coefficients, with each of the coefficients also needing to be quantified with precision, i.e. it can only be achieved at the cost of a large number of elementary operations on bits.
Known filtering and decimation solutions tend rather to subdivide the filtering function into two or three stages. This provides advantages in terms of overall complexity, since the number of operations performed in each stage is significantly reduced. However, in those known solutions, the configuration of the various successive stages is specific to the carrier and to the data rate of the channel that is to be extracted. In other words, at each stage, filtering is adapted to the characteristics (carrier and data rate) of a particular channel. Consequently, as a general rule, two successive stages are not identical. In addition, a given filtering and decimation apparatus is not simple to use for two distinct channels of interest. Under such circumstances, all of the filtering and decimation parameters in each stage need to be updated individually.
In other words, those known solutions for filtering and decimation suffer from the major drawback of requiring large capacity storage for storing several sets of filtering and decimation coefficients, and also of requiring a mechanism for selecting between pertinent sets of coefficients. In addition, since the stages are completely different from one another, there is no possibility of implementing them by sharing a common computation resource.
Elsewhere, for the problem of transposition, a commonly adopted solution consists in transposing the entire multiplex once so as to center the channel of interest on zero frequency. In this way, the above-mentioned known solutions for filtering and decimation in which filtering is centered likewise on zero frequency, can be used to extract the channel of interest and to decimate it.
Such transposition is not very advantageous. It needs to be performed immediately after the analog-to-digital converter, and therefore at the high sampling frequency that is imposed thereby. Unfortunately it would be more advantageous to minimize the sampling frequency before proceeding with transposition into baseband. It is recalled that the values that are accessible by frequency transposition are, by their very nature, quantified, and that as a result, for given precision, the fineness of such transposition decreases with increasing sampling frequency. This precision is represented by the size of the memory for storing the accessible transposition values. In addition, since the band is still complete at this stage, the width of the frequency range to which the transposition must be applied is at its maximum.
A particular object of the invention is to mitigate those drawbacks of the prior art.
More precisely, an object of the present invention is to provide a digital method and apparatus for filtering, decimating, and transposing into baseband, which method and apparatus are of hardware cost that is reduced compared with the above-mentioned known solutions.
Another object of the invention is to provide such apparatus and such a method that impose no constraint on the frequency multiplex (and in particular do not require any particular distribution of channel carriers within the multiplex).
Another object of the invention is to provide such apparatus and such a method in which it is possible for some of the stages to be identical in terms of design structure and hardware.
A further object of the invention is to provide such apparatus and such a method in which the number and the configuration of the various stages are not specific either to the carrier or to the data rate of each channel that is to be extracted, but depend solely on the initial and final oversampling factors.
These various objects, and others that appear below, are achieved by the invention by means of apparatus for filtering, decimating and transposing into baseband, the apparatus being of entirely digital type, and serving both to extract and transpose into baseband one of the channels, referred to as the xe2x80x9cchannel of interestxe2x80x9d, of a frequency multiplex of channels, and also to reduce the oversampling factor of said channel of interest, each channel being associated with a distinct carrier,
the said apparatus comprising at least two stages in cascade, each stage comprising filter means and decimation means, said filter means being placed upstream from said decimation means and enabling a portion of the spectrum to be purged at locations where the decimation means produce aliasing,
the apparatus being characterized in that each stage also comprises transposition means placed upstream from said filter means and associated with a transposition frequency previously selected from a set of N possible distinct transposition frequencies so as to transpose the carrier associated with said channel of interest as close as possible to zero frequency.
The general principle of the invention thus consists in sharing frequency transposition between the various stages of the structure. Thus, the channel of interest is transposed into baseband in successive steps, and each transposition step can be wide. The transposition means of each stage can therefore be less complex than when transposition is performed on a single occasion only.
If necessary, the last stage can be followed by fine transposition means so as to perfect transposition into baseband.
The invention puts no constraints on the frequency multiplex. In other words, the carrier and the data rate of the channel of interest can be arbitrary. The transposition performed at the input to each stage ensures that the channel of interest is correctly filtered (i.e. without any information being lost).
The invention also makes it possible to use transposition means that are not very complex because of the synergy that exists between the transposition performed in a given stage and the decimation performed in the preceding stage. Because of the decimation performed in the preceding stage, the sampling frequency of a given stage is less than that of the preceding stage. Consequently, for the same granularity of transposition (i.e. for the same difference between two transposition frequencies, expressed as a function of the sampling frequency), the transposition performed at a given stage is finer than that performed at the preceding stage. In other words, decimation produces a magnifying-glass effect which avoids the need to make transposition more and more precise on going from stage to stage.
Advantageously, said decimation means are identical in all of the stages. In this way, the channel of interest can be changed without modifying the decimation means.
Advantageously, the decimation means of each stage have a decimation factor equal to two. Thus, the filtering performed at each stage does not need to be very powerful.
Preferably, said filter means perform half-band lowpass filtering. Thus, the invention can be implemented using half-band filters, which are the simplest filters to make.
Advantageously, the filter means of a given stage perform half-band lowpass filtering with a transition zone that is at least as narrow as that of the half-band lowpass filtering performed by the filter means of the stage preceding said given stage.
In this way, the filters can be simpler the closer they are to the beginning of the system, and this point is advantageous since the early stages operate at higher speeds than the later stages. In other words, this makes it possible for the filters in the earlier stages to relax specifications for oscillation in the band, and thus makes it possible to reduce the number of coefficients they use.
In a preferred embodiment of the invention, said transposition means of each stage perform complex multiplication by a transposition coefficient:
C=exe2x88x92jxc2x72xcfx80xc2x7ftxc2x7kTe
ft being said transposition frequency of said stage as previously selected from a set of N possible distinct transposition frequencies so as to transpose the carrier associated with said channel of interest as close as possible to zero frequency; and
kTe being the case sampling instant, k being an integer that increases with time, and Te=1/fe with fe being the sampling frequency of said stage.
It is recalled that the fineness of the transposition performed in a stage increases with increasing number N of possible distinct transposition frequencies (and thus with deceasing transposition granularity).
Advantageously, said transposition frequency ft of said stage is written: ft=(lxc2x7fe)/N, where l is an integer lying in the range 0 to Nxe2x88x921.
In an advantageous variant, said transposition frequency ft of said stage is written: ft=(fe/(2N))xc2x7(2l+1), where l is an integer lying in the range 0 to Nxe2x88x921.
It should be observed that in the above two cases, the granularity of the transposition performed at each stage is equal to fe/n, and the transposition coefficient C can take on N distinct values (modulo N) for N successive sampling instants.
Preferably, at least two successive stages, including the first stage, are identical, with only said transposition frequency being specific to each stage, and said at least two successive identical stages are implemented using a single hardware cell whose transposition, filter, and decimation means are shared by said at least two successive identical stages.
The term xe2x80x9cidentical stagesxe2x80x9d is used to mean stages having transposition means, filter means, and decimation means that are identical. For the transposition means, this implies that the sets of N possible transposition frequencies of the various stages are identical. Naturally, this does not mean that the selected transposition frequency is the same for all of the identical stages.
Thus, by sharing hardware resources for implementing identical stages, a significant saving in computation power is achieved (and thus in ASIC surface area). All identical stages are implemented using a single hardware cell that operates fast enough to be able to perform all of the processing in parallel. This parallel processing is made possible by the fact that each successive stage operates more slowly than the preceding stage (because of the decimation).
Preferably, the fineness of the transposition performed by the transposition means of a given stage is greater than or equal to the fineness of the transposition performed by the transposition means of the stage preceding said given stage.
In this way, transposition can be simpler closer to the start of the system, and this point is advantageous since the early stages operate faster than the later stages. In other words, transpositions that require more computation (i.e. that are finer) are performed at slower operating speeds (or sampling frequencies).
Advantageously, as a function of the initial and final oversampling factors of said channel of interest, respectively at the input to and at the output from said apparatus, said frequency multiplex of channels is applied directly to an appropriate stage situated downstream from the first stage.
In other words, depending on the initial amount of oversampling, it can be decided to avoid passing the signal through one or more of the first stages, by passing it directly into an intermediate stage.
The invention also provides a method of filtering, decimating and transposing into baseband, the method being of the entirely digital type and enabling both a particular channel referred to as the xe2x80x9cchannel of interestxe2x80x9d in a frequency multiplex of channels to be extracted and transposed into baseband, and also the oversampling factor of said channel of interest to be reduced, each channel being associated with a distinct carrier,
the method being characterized in that the operations of filtering, decimation, and transposition into baseband are performed in at least two successive processing stages where each processing stage comprises the following successive steps:
transposition enabling the carrier associated with said channel of interest to be transposed as close as possible to zero frequency;
filtering; and
decimation, said filtering step serving to purge a portion of the spectrum in locations where the decimation step produces aliasing.
Advantageously, said method comprises a preliminary stage of determining the number of successive processing stages and the transposition, filter, and decimation characteristics of each of said processing stages, the decimation factor of each stage being assumed to be known, the initial oversampling factor input to the first stage being assumed to be known, a maximum oversampling factor at the output from the last stage being imposed,
said preliminary stage comprising the following steps:
with respect to the last processing stage: the optimum transposition characteristics (in terms of fineness of transposition) and filtering characteristics (in terms of difference from ideal filtering) are determined that enable said imposed maximum oversampling factor at the output from said last processing stage to be obtained;
with respect to the penultimate processing stage:
on the basis of the decimation factor of the last processing stage and on the basis of the maximum oversampling factor imposed at the output from the last processing stage, a maximum oversampling factor required at the output from said penultimate processing stage is calculated; and
the optimum transposition characteristics and filter characteristics are determined enabling said maximum oversampling factor required at the output from said penultimate processing stage to be obtained;
the same reasoning is repeated until an earlier processing stage referred to as the xe2x80x9cfirstxe2x80x9d processing stage is obtained such that when said initial oversampling factor is applied thereto, then the oversampling factor at the output from said first processing stage is less than that previously calculated maximum required oversampling factor.
This prior stage makes it possible to optimize the filters of the various stages individually, starting with the last stage and working back towards the first.
Preferably, said preliminary stage also comprises a final step consisting in:
for the last or the last X processing stages where Xxe2x89xa71: retaining the optimum transposition and filter characteristics specific to each processing stage; and
for each of the L-X first processing stages, where L is the total number of processing stages and L-Xxe2x89xa72: replacing the optimum transposition of filter characteristics specific to each processing stage with common transposition of filter characteristics equal to those of the L-Xth processing stage.
It appears that from a certain stage (the L-Xth in the present notation) and going back to the first stage, the filtering is extremely simple to perform so there is little to be gained in terms of complexity by optimizing each filter stage individually. Under such circumstances, it is preferable to make the filter stages identical, thus making it possible to implement all of them using a single hardware cell.
The invention also provides a digital demodulator of the type comprising at least one single channel demodulation system, with each system including apparatus as specified above for filtering, decimating, and transposing into baseband.
Other characteristics and advantages of the invention appear on reading the following description of two preferred embodiments of the invention, given by way of non-limiting example and with reference to the accompanying drawings, in which: